Manufacture of chromates



Patented Feb. 20, 1934 P'ATEN'U OFFICE.

MANUFACTURE OF CHROMATES Omar F. Tarr, Stoneleigh, Md., assignor to Mutual Chemical, Company of America, New York, N; Y., a corporation of- New Jersey No Drawing. Application March 30, 1932 Serial No. 602,105

13 Claims. (01; 23-56) This invention relates to the production of soluble chromates from chrome ore and provides an improved process of manufacturing alkali chromates from such chromium bearing materials.

According to this invention, the chromium is removed from the ore by a multi-stage roasting and extraction process in the first stage of which fresh ore is roasted under oxidizing conditions 10.- with an alkaline earth refractory material such goetc. which are present are so controlled that throughout the process the fusion of the charge and formation of'hard' infusible lumps or balls is prevented even though temperatures above 1000 and as high as 1200 to 1400 C. are employed. Because the fusion of. the charge is prevented, the process can be readily carried out in a mechanically operated furnace. Rotary kilns may be used to advantage and are particularly desirable because of the low labor cost involved in their operation, and the small" initial cost and upkeep.

When the usual chrome ore mixtures are roasted in rotary kilns or other mechanically operated furnaces where the charge is rather violently agitated during its passage through the furnace, it is found that the charge fuses at least to some extent and the material is discharged in the form of lumps or balls, etc. The fusion of easily fusible mixtures of. chrome ore containing an alkali base takes place quickly and at temperatures well below the active oxidizing temperature of the chromium present. On continued passage of such an easily fusible charge of material through a rotary kiln, with increase in temperature the mass hardens to form lumps or balls which are able to withstand the roasting temperature without again softening. This hardening after the first softening stage is probably due to the formation of various compounds of the alkali base with impurities present in the ore such as alumina, silica, etc, which compounds are relatively infusible. The formation of these compounds removes a, portion of the fusible alkali base from the system before the chromium has been actively attacked with the result that the resulting mass which, isno, longer easily fusible contains. a highpercentage of unattacked chromium. The hardened coated lumps are quite. imperviousto the oxidizing atmosphere of the furnace and even with long continued exposure. to the atmosphere at oxidizing, temperature the ore is only slightly attacked. H

Any compounds of the alkali base with. alumina, silica, etc. which may form in furnace charges which comprise both an alkali base and an alkaline earth material in addition to the chromium are decomposed at higher tempera: turesby the formation of alkaline earth cornpounds making the alkali base again available for combination with chromium The presence of such compounds therefore might be beneficial if the charge could be kept in a porous condition and relatively free from infusible lumps.

If enough basis refractory material such as lime is added to the charge to pre'ventfusion, the, amount of material to be treated in later stages of the, roasting process is, increased with consequent increase in the drying, grinding and fuel costs as well as the. increased cost of raw material, and the efficiency of recovery is lowered particularly in the final stage of the roasting process due to the hi h ratio, of the basic refrace tory material present. If on the other hand, the, amount of alkali base present is decreased in order to prevent fusion of the charge, the amount of chromate extracted in the first stage of the process will be decreased, and it will be necessary to, increase the number of stages of the roasting treatment in order to get a high recovery of chromium from the ore. The other alternative for preventing fusion has been to lower the roasting temperature but this materially increases the time of treatment necessary to obtain a good commercial yield of chromium from the ore.

According to this invention the porosity of the furnace charge is maintained and the formation of hard infusible lumpsor balls is prevented even at high temperatures by removing alkali base from the system at temperatures below the fusion temperature by forming less fusible compounds from it. By adding to the furnace charge a part of the leached residue from a previous roasting operation, part of the alkali base is removed from the system. by reaction with insoluble chromate present in the residue. This reaction takes place at a low temperature. After removing a required amount of the alkali base from the charge below its fusion temperature by the formation of alkali chromate, the charge may be heated to a high temperature without fusing.

By adding a part of the leached residue from a previous roasting and extracting step to a mixture of chrome ore, alkaline earth material and an alkali base, I obtain a furnace charge which can be roasted at temperatures over 1000 C. and up to 1200 or 1400" C. without fusing. The absence of any tendency in the mixture to fuse may be attributed to the presence of insoluble chromate in the leached residue. chromate is attacked by the alkali base at a low temperature and thus a portion of the alkali base is removed from the system. Less alkali base is added torthe charge than that theoretically required to convert all of the chromium present to chromate. Due to the absence of fusion and the formation of lumps and balls the chromium is oxidized to chromates, both soluble and insoluble without fusing. The insoluble chromate is readily converted to soluble chromate in a subsequent roasting operation and its presence in the residue returned for roasting with fresh chrome ore is important. The higher roasting temperatures made possible by this invention are desirable because they bring about the rapid formachromium into soluble chromate.

tion of soluble chromate both by the conversion of insoluble chromate to soluble alkali chromate and by the direct attack of alkali base on the ore. I-Iigh temperatures also are favorable for the oxidation of a large part of the remaining chromium maining in theresidue will be present as waterinsoluble chromate. In the final stage of the roasting operation, I add sufficient alkali to convert this insoluble chromate to alkali chromate which is water-soluble, and also sufiicient alkali to recover the remaining chromium as watersoluble chromate.

By continuously returning to the first stage of the roasting operation, the same quantity of leached residue from a previous first-stage roast, the desired infus bility'of the charge is maintained without adding inert material which would increase the amount of material to be treated in the second stage of the roasting operation, which would be the case if any other fusion preventive were used.

In a two-stage roasting operation the amount of alkali base employed in the first step is more than half the amount theoretically required for the chromium present in the ore because the removal of chromate by the leaching of the first furnaceproduct reduces the amount of material subjected to the second stage of the roasting operation and the amount of alkali base which can beemployed without fusion is thus necessarily less than theamount of alkali base used in the first step of the process In the product usually obtained from the first-stage roast no uncombined alkali base is present. However, this is not usually the case in the final stage, where a deficiency of alkali would result in a loss of chromium to the process.

The percent of the total alkali base theoretically required for the conversion of all of the chromium to chromate which is used in the first stage of a two-stage recovery process and the amount of alkaline earth refractory material employed This insoluble may vary. The amount of residue may also vary. It alone makes a very convenient and effective variable for controlling the fusibility. The following proportions have been found suitable for treating an ore containing ClzOs:

' Parts Chrome ore 100 Leached residue from preceding first-stage roast 90 'Burned lime 50 Soda ash 50 This charge contains somewhat over 70% of the amount of soda ash theoretically required for combining with the chromium present in the ore and about of the amount required for all the chromium present. The mixture is roasted in a rotary kiln for about an hour at a temperature of 1200 to 1400 C. in an oxidizing atmosphere when all of the soda present in the roast will be combined with the chromium as chromate and a further quantity of chromium will be oxidized to insoluble chromate, for example calcium chromate. The furnace product is then leached for the removal of soluble sodium chromate. The residue is then dried and ground. 90' parts of the leached residue which retains the insoluble chromates is reused in admixture with fresh chrome ore in a succeeding first-stage roast. The remainder of the residue which contains about 21 parts of CaO is mixed with soda ash using about 11.9 parts of sodium carbonate to parts of residue. It is customary in the last roasting step to use approximately the amount of alkali base theoretically required for combination with the chromium present. This charge is also roasted in a rotary kiln for about an hour at l200 to 1400* C. It is not necessary to add lime or other refractory material to prevent fusing.

The above proportions of materials have been found satisfactory in roasting ores of varying iron content. Ores analyzing as low as 11% FeO and as high as 20% FeO have been roasted in this way with entire satisfaction. With ores of higher or lower chromium content the amount of alkali base employed is advantageously altered proportionately so that the amount of alkali base is about 70% of the amount theoretically required for combining with the chromate. With an increase or decrease in the amount of alkali base employed, the amount of residue employed is varied accordingly to maintain the desired infusibility of the charge. The process is applicable to ores varying widely in FeO, A1203, SiOz and MgO content.

The invention not only uses less raw materials, thus reducing the raw material cost and also the labor and treating costs due to a reduction in the amount of material handled, but by reducing the roasting time, the fuel cost is materially reduced and a larger return is realized on the invested capital. 7

I claim:

1. A method of roasting chrome ore while preventing fusion of the charge and the formation of fused balls or lumps which comprisesroasting with the chrome ore an alkali base and basic refractory material including a considerable amount of alkaline earth chromate, forming alkali chromate from said alkaline earth chromate at a low temperature, and then raising the temperature of the charge to convert compounds of alkali metal with alumina and silica to alkaice line earth compounds and using sufiicient refractory material in the charge to maintain the whole mass in a porous condition while roasting at temperatures over about 1000 C.

2. The method of roasting chrome ore in a rotary kiln which comprises roasting with the ore, an alkali base and basic refractory material including a considerable amount of alkaline earth chromate, forming alkali chromate from said alkaline earth chromate at a low temperature, gradually raising the temperature of the charge thereby converting compounds of alkali metal with aluminum and silica to alkaline earth compounds of aluminum and silicon, and adding sufficient refractory material to the charge to maintain the whole mass in a porous condition throughout the roasting operation while raising the roasting temperature to as high as 1200" C.

3. A method of roasting chrome ore in a rotary kiln which comprises roasting chrome ore, basic refractory material including a considerable amount of alkaline earth chromate and over 50% of the alkali base theoretically required to combine with the chromium present, forming alkali chromate from said alkaline earth chromate at a low temperature, gradually raising the temperture of the charge thereby converting compounds of alkali metal with alumina and silica to alkaline earth compounds of aluminum and silicon and using sufficient refractory material in the charge to maintain the whole mass in a porous condition while gradually raising the temperature and eventually heating the charge to 1200 C.

4. The method of roasting chrome ore in a rotary kiln in a plurality of stages using only lime and alkali base with the ore which comprises in the first stage roasting chrome ore, lime, a portion of the leached residue from a similar roasting of the same amount of ore and alkali base, but not sufficient alkali base to combine with all the chromium, all in such proportions as to prevent the formation of a mass fusible at the roasting temperature and eventually heating the charge to 1200 C.

5. The method of roasting chrome ore in a rotary kiln in two stages using only lime and alkali base with the ore which comprises roasting chrome ore, lime, sodium carbonate and a portion of the leached residue from a similar previous roasting of the same amount of ore in such proportions as to prevent the formation of a mass fusible at the roasting temperature, leaching the furnace product thus produced and roasting at least a part of the leached residue in admixture with an alkali base by heating to a temperature above 1200 C. in a rotary kiln without fusing.

6. The method of roasting chrome ore which comprises roasting to a temperature above 1000 C. in a rotary kiln a mixture comprising 100 parts of chrome ore, 50 to 70 percent of the amount of alkali base theoretically required to combine with the chromium in the ore to form alkali chromate, about 50 parts of a basic refractory material and about 90 parts of leached residue from the roasting of a similar charge.

7. The method of roasting chrome ore which comprises roasting a mixture comprising 100 parts of ore with about 50% CizOs content, about 50 parts of basic refractory material, about 50 to 70% of the amount of alkali base theoretically required to combine with the chromium present in the ore and about 90 parts of the leached residue from the previous roasting of a similar charge.

8. The method of roasting chrome ore which comprises roasting to a temperature not less than 1000 C., a mixture of 100 parts of chrome ore, about 50 parts of a basic refractory material, from 50 to 70% of the amount of alkali base theoretically required to combine with the chromium present in the ore and leached residue from the previous roasting of chrome ore, an alkali base and alkaline earth material.

9. The method of roasting chrome ore to a temperature above 1200 C. which comprises roasting a mixture of 100 parts chrome ore, about 50 parts of alkaline earth refractory material, between 50 to 70% of the amount of an alkali base theoretically required to form chromate from the chromium in the ore and sufficient leached residue from the previous roasting of a previous similar charge to prevent fusion of the furnace charge at any time. during the roasting operation.

10. The method of roasting chrome ore using only an alkaline earth refractory material and alkali base with the ore which comprises progressively heating a mixture of chrome ore, alkaline earth refractory material, alkali base and an insoluble chromate, converting the insoluble chromate to alkali chromate and then as the temperature is increased forming alkaline earth chromate from chromium in the ore and heating the mass above 1200 C. without fusing.

11. The method of roasting chrome ore using only an alkaline earth refractory material and alkali base with the ore which comprises progressively heating a mixture of chrome ore, alkaline earth refractory material, alkali base and leached residue from a similar roasting operation thereby converting insoluble chromate present in the residue to alkali chromate and as the temperature is increased forming alkaline earth chromate from chromium in the ore and heating the mass to a temperature above 1200 C. without fusing.

12. The method of roasting chrome ore in a rotary kiln without forming fused lumps or balls in the charge using only lime and alkali base with the ore which comprises progressively heating a mixture of chrome ore, lime, alkali base and leached residue from a previous similar roasting operation so as to convert insoluble chromate in the residue to alkali chromate at a low temperature thereby removing alkali base from the system and then as the temperature is increased forming more alkali chromate and calcium chromate from chromium in the ore and eventually heating the mass to over 1200 C. without fusing.

13. The method of roasting chrome ore which comprises roasting a mixture comprising 100 parts of chrome ore, about 50 parts of basic refractory material, about 50 to 70% of the amount of alkali base theoretically required to combine with the chromium present in the ore, and about 90 parts of the leached residue from the previous roasting of a similar charge.

OMAR F. TARR. 

